The present invention relates to a lens grinding apparatus which is used to grind the periphery of an eyeglass lens, more particularly, to an apparatus suitable for grinding the angular edge portions of the eyeglass lens.
An apparatus is known in the art which grinds an eyeglass lens so that it fits into an eyeglasses frame. In an optician's shop, an optician processes the periphery of each eyeglass lens so that it is provided with a bevel or a groove which fit into an eyeglasses frame selected by a customer and he then mounts the processed lens into the frame.
The thus ground lens has an angular portion at both front and rear ends of the edge. If such angular portions are left intact, they may possibly hurt the user or become a cause of breakage or other damage of the lens. Therefore, it is common practice for lens processors to chamfer the angular edge portions of the lens.
Conventionally, chamfering is performed with a hand grinder having a rotating grinding wheel bounded by conical slopes and the optician who holds a lens urges its edge into contact with the chamfering wheel and chamfers the angular edge portions to give the desired shape under visual checking.
However, chamfering with the hand grinder requires skill and is not easy to accomplish in a manner to give satisfactory results. For an unskilled optician, the operation is time-consuming and does not guarantee grinding to the intended shape. In addition, considerable burden is imposed on the optician.
Under the circumstances, it has been proposed that the lens grinding apparatus be provided with a chamfering capability so that chamfering can be effected automatically. In this type of apparatus, the lens held on a rotating shaft and the chamfering wheel are both rotated and the edge of the lens is chamfered while effecting control in such a way as to change the edge position of the lens relative to the chamfering wheel's surface.
For successful control of the chamfering operation, it is necessary that the three-dimensional positions of the lens rotating shaft and the chamfering wheel be determined to satisfy an appropriate relationship. However, if a sufficient allowance is provided with a view to ensuring that the chamfering wheel will not interfere with the lens rotating shaft, the smallest diameter of the lens that can be processed increases, making it difficult to process lenses of small minimum diameter such as a pince-nez.
In addition, a lens having a negative power increases in edge thickness with increasing diopter. Further, the edge thickness varies from one area to another in accordance with its shape. If such lenses are mounted on an eyeglasses frame having a comparatively thin rim, the edge of each lens projects so much from the rim of the eyeglasses frame that the customer often refuses to wear the eyeglasses because they are unseemly or have other defects.
Speaking of rimless eyeglasses which have recently gained popularity among users, the entire part of the edge portions is visible to the outside, making the aesthetic appeal of the eyeglasses more important.